This invention relates to methods and systems for treating dusts that are generated in steel production, the recycling of scrap metal, and other metallurgical operations. In particular, this invention relates to methods and systems for recovering lead, cadmium, and zinc from baghouse dust that is generated in steel production by electric arc furnace minimills. The methods and systems of the present invention recycle the dust to a form which can be further processed by the steel mill, and allow for the recovery of valuable components in the dust.
Baghouse dust is a mixture of metal oxides that are collected by scrubbers, electrostatic precipitators, bag filters, or other known filtering systems, in electric arc furnace (EAF) and blast furnace steel-making facilities and other iron-making plants. The dust, also called EAF dust, typically is composed mainly of oxides of iron, zinc, lead, tin, cadmium, chromium, manganese, nickel, copper, and molybdenum. Silica, lime and alumina may also be present in the dust.
Increasing levels of zinc in scrap steel due to the increased use of galvanized materials in automobile manufacture and keener competition for zinc free scrap have contributed to increased dust production. According to recent estimates, there were an estimated 600,000 tons of EAF dust generated from U.S. carbon steel operations in 1992. EAF operators are paying an average processing fee of $150 to $200 per ton of dust. Electric Arc Furnace Dust--1993 Overview, CMP Report No. 93-1, Arthur D. Little, Inc., July 1993. Annual disposal expenditures are said to approach $120 million for the industry.
The approaches that have been reportedly considered thus far for recycling or disposing of EAF dust fall into the following three general categories:
1) Briquetting, pelletizing or otherwise fixating the dust in a leachproof matrix and storing or disposing of the fixated product; PA1 2) Reducing the dust with coal, methane or hydrogen at an elevated temperature and separating condensable zinc vapor from a nonvolatile slag, e.g., using a plasma furnace or a flame reactor process; PA1 3) Removing the zinc by a hydrometallurgical process.
The first approach is not favored because it generates a disposable but nonsalable product. Furthermore, the dumping of untreated EAF dust in municipal landfills was banned in about 1988 under the Resource Conservation and Recovery Act (RCRA) due to the presence of leachable hazardous elements including lead, cadmium and chromium, which may enter the groundwater system and contaminate drinking water.
The majority of dust is presently treated by a thermal reduction process known as High Temperature Metal Recovery (HTMR) processing whereby the dust is transported as a hazardous material to an off-site processor for thermal treatment and removal of zinc and other metals. In the HTMR process known as the Waelz Kiln Process, which is practiced by Horsehead Resource Development Company, Inc. ("Horsehead") and is the most commercially successfully process known to date for treating EAF dust, the EAF dust, other wastes, coke or coal, lime and silica are mixed and fed to a rotary kiln furnace. The furnace is maintained at about 1100.degree. to 1200.degree. C. The zinc and other volatile non-ferrous metals in the feed are vaporized in the furnace off-gas and are carried from the furnace to an external dust collection system consisting of a cyclone and a baghouse. The resulting Waelz oxide is a crude zinc bearing product which is further refined by either a second kiln step where the material is further heated and sintered to form a zinc clinker material which is suitable for use in an electrothermal zinc furnace or is hot briquetted for use in an Imperial Smelting Furnace. Horsehead utilizes the former option and ships the resulting zinc product to Zinc Corporation of America, while Berzelius employs the latter option. A lead/cadmium by-product is shipped to another facility where the cadmium is recovered as a metal and the lead is recovered as lead sulfate which is then sent to a lead smelter. The other product of the furnacing operations is an iron rich slag which is considered suitable for road building applications.
The Waelz Kiln Process used by Horsehead is further described in S. E. James and C. O. Bounds, Recycling Lead and Cadmium, As Well as Zinc, From EAF Dust, in Lead-Zinc '90, Edited by Mackey and Prengaman, The Mineral, Metals & Materials Society, 1990, incorporated herein by reference in its entirety. Other variations of the Waelz Kiln Process are described in R. Kola, The Processing of Steelworks Waste, Lead-Zinc '90, Edited by Mackey and Prengaman, The Mineral, Metals & Materials Society, 1990; and N. Tsuneyama, M. Takewaki, and M. Yasukawa, Production of Zinc Oxide For Zinc Smelting Process From EAF Dust At Shisaka Works, Lead-Zinc '90, Edited by Mackey and Prengaman, The Mineral, Metals & Materials Society, 1990, all of which are incorporated herein by reference in their entireties.
Other examples of HTMR processes are described in Electric Arc Furnace Dust--1993 Overview, CMP Report No. 93-1, Arthur D. Little, Inc., July 1993 and briefly mentioned herein. The "HTR Process" utilizes a modified Waelz Kiln Process at 1400.degree. C. In the HTR Process, zinc and other non-ferrous metals are volatilized and collected in a cyclone and baghouse. The solids exiting the kiln contain iron oxide and are sent to a landfill for burying. The "Zia Inclined Rotary Reduction System" uses a modified rotary kiln fired with oxy/gas burners and containing a bellied kiln. The zinc and lead metallic vapors leaving the furnace are recovered in a splash condenser. In the "INMETCO Process" pelletized material is fed into a rotary hearth furnace at 1225.degree. C. where the pellets are sintered and several metals, including zinc, are vaporized. The vaporized metals are collected in a wet scrubber system as a sludge. The non-volatile metals and the sintered slag are fed into a second furnace where the metal is smelted to produce an iron nickel chromium alloy.
The second (thermal reduction) approaches, including the Horsehead process, are problematic in that they necessitate costly, rather large, thermally insulated facilities and considerable energy expenditures, which render the technology impractical and cost ineffective for on-site treatment at most steel mini mills. Thus, the EAF operator incurs the costs, risks, and increased liability of shipping the dust as a hazardous waste to a regional treatment center. Neither the HTR or INMETCO process addresses the disposal of collected secondary dust.
The hydrometallurgical processes that are widely used for recovering zinc from iron-containing ores typically include selective dissolution (leaching), precipitation, filtration, and washing. Such processes are described in U.S. Pat. No. 5,028,410 ("Spink et al."), F. G. Prado, J. P. Dempsey, and B. W. Wiegers, High Purity Zinc Oxide Production From Residues In Automobile Scrap Recycling, Symposium on Recycle And Secondary Recovery Of Metals, The Minerals, Metals, & Materials Society, 1985, pp. 183-93, F. Prado, High Purity Zinc Oxide From A Wide Range Of Industrial Residues, Second Intern. Symposium Recycling--of Metal and Engineered Materials, Edited by van Linden, Stewart, Jr., and Sahai, The Minerals, Metals & Materials Society, 1990, F. G. Prado and F. L. Prado, EAF Dusts: A Viable Complete Minimization, Extraction and Processing for the Treatment and Minimization of Wastes, Edited by Hager, Hansen, Imrie, Pusatori, and Ramachandran, The Minerals, Metals & Materials Society, 1993, the disclosures of which are hereby incorporated by reference in their entireties.
Although hydrometallurgical approaches theoretically involve the least costly equipment and the least expenditures of energy, such processes generate excessive amounts of environmentally objectionable effluents and often require processing conditions which pose potential safety and health hazards to the worker. In addition, EAF dusts typically contain a substantial portion of zinc in the form of zinc ferrite (ZnFe.sub.2 O.sub.4) which cannot easily be decomposed into separate iron and zinc constituents and which is substantially insoluble in most solvents. Thus, another problem with hydrometallurgical approaches is that it is often difficult to achieve separation of zinc from zinc ferrite without using extreme process conditions (solvents, temperature, pH) which also tend to dissolve unwanted contaminants which further interfere with the treatment process. As a result, the percentage recovery of zinc by hydrometallurgical processes tends to be rather low.
For example, in U.S. Pat. No. 4,071,357 ("Peters"), Peters describes a leaching method to recover zinc oxide from steel-making flue dust using ammonia and carbon dioxide. In Table I, Peters reports that only 54.7% of the zinc from the dust went into solution after 4 hours of leaching. Thus, a substantial portion of the zinc remained in the leach residue. In U.S. Pat. No. 5,204,084 ("Robinson et al."), Robinson reports that only 61.2% of the zinc in a roasted zinc sulphide concentrate was extracted in an ammonia ammonium carbonate solution.
In an article by Nyirenda et al., Ammonium Carbonate Leaching of Reduced Electric Arc Furnace (EAF) Dust, Residues and Effluents--Processing and Environmental Considerations, Edited by Reddy, Imrie, and Queneau, The Minerals, Metals & Materials Society, 1991, the authors heated a sample of EAF dust in a furnace at 650.degree. C. using a mixture of CO.sub.2 /CO gas for 90 minutes. The reduced product was then leached using ammonia ammonium carbonate solution. The solution dissolved 70% of the zinc and 25% of the iron. Nyirenda also reduced a mixture of pure zinc oxide and pure wustite at the same conditions and then leached the reduced product. He reported that 80% of the zinc and 10% of the iron entered the solution.
A method for treating neutral leach residues using an Ausmelt submerged lance reactor is described by Altepeter and James in Proposed Treatment Of Neutral Leach Residue At Big River Zinc, Residues and Effluents--Processing and Environmental Considerations, Edited by Reddy, Imrie, and Queneau, The Minerals, Metals & Materials Society, 1991. In this method, moist residue is smelted in an agitated slag bath, and then coal is added to the slag bath which is then reduced at 1400.degree. C. The zinc rich fume was collected in a baghouse and leached with sulfuric acid.
Accordingly, it is an object of this invention to provide methods and systems for the treatment of raw materials, especially EAF dust, which separates and recycles lead, cadmium, zinc, and iron, in commercially useable forms, and which generates substantially no environmentally objectionable waste stream.
It is another object of this invention to provide methods and systems which separate and recover substantially all of the zinc from the zinc ferrites and zinc oxides in EAF dust.
It is another object of this invention to provide methods for separating and recycling zinc from EAF dust, whereby zinc is recovered in the form of zinc oxide.
It is another object of this invention to provide methods and systems for the treatment of EAF dust which can be utilized on or adjacent to the premises of a steel making mill, thereby avoiding the need to transfer the EAF dust to a separate off-site location, and reducing the EAF operator's costs and potential liability associated with transporting hazardous materials.